Energy recovery twin linear $e^+e^-$, $e^-e^-$ colliders (ERLC ) with high luminosities and accelerating gradients

TL;DR

The paper develops and analyzes energy-recovery twin superconducting linear colliders (ERLC) for both $e^+e^-$ and $e^-e^-$ modes, leveraging twin RF structures to eliminate parasitic linac collisions and enable repeated beam use. It shows that in pulsed operation the $e^+e^-$ luminosity is largely gradient-independent for fixed power and scales as $L \propto Q_0^{1/2}$, while CW operation requires a threshold power with $L \propto Q_0^{1/4}$; the $e^-e^-$ option, though simpler due to single-use bunches, exhibits a similar weak dependence on $Q_0$ and independence from $G$. The authors derive and summarize luminosity formulas for ideal, pulsed, and CW regimes of both collider types, and provide example parameter sets suggesting that with $G\approx40$ MeV/m and using Nb$_3$Sn or traveling-wave cavities, $L_{e^+e^-} \sim$ (1–2.5)×$10^{36}$ cm$^{-2}$s$^{-1}$ and $L_{e^-e^-} \sim$ (3–7)×$10^{36}$ cm$^{-2}$s$^{-1}$ can be achieved at $2E_0=250$–$500$ GeV with total wall powers of 150–300 MW. The work identifies Nb$_3$Sn and TW structures as promising technologies and discusses the many engineering challenges that must be addressed before such machines could become Higgs factories.

Abstract

Recently, the $e^+e^-$ high-energy superconducting linear collider with energy recovery (ERLC) and multiple use of beams has been proposed, using twin RF structures to avoid parasitic collisions in linacs. Such a collider can operate in either pulsed (with duty factor DF) or CW (continuous) mode, if sufficient power is available, with a luminosity of ${\cal O}(10^{36})$ cm$^{-2}$s$^{-1}$ at $2E_0=$250-500 GeV. In this paper, we point out that the luminosity of the ERLC operating in the pulsed mode does not depend on the accelerating gradient G (for the same total power), allowing the ERLC to operate with the highest available accelerating gradients. This is also true for the CW mode at a power significantly exceeding the threshold power for this mode. The $e^+e^-$ luminosity depends on the quality factor of cavities as $L\propto Q_0^{1/2}$. We also consider for the first time a twin $e^-e^-$ collider with energy recovery and estimate its achievable luminosity. Such an $e^-e^-$ collider is simpler than an $e^+e^-$ one. It does not require beam recycling since electrons can be produced anew each time. The $e^-e^-$ luminosity depends on the quality factor as $L\propto Q_0^{1/4}$. RF travelling-wave structures allow higher gradients and reduced thermal loading. It is shown that an ERLC with an accelerating gradient of $G=40$ MeV/m can be operated in CW mode with $L_{e^+e^-} =$ (1-2.5)\,$10^{36}$ and $L_{e^-e^-} =$ (3-7)$10^{36}$ cm$^{-2}$s$^{-1}$ for reasonable total powers of 150 and 300 MW at $2E_0=250$ and 500 GeV, respectively. Such a machine is a promising candidate for a Higgs factory.

Figures (4)

• Figure 1: The layout of the SC twin $e^+e^-$ linear collider Telnov-erlc
• Figure 2: The layout of the SC twin $e^-e^-$ linear collider.
• Figure 3: Dependence of the $e^+e^-$ luminosity on the total power for $2E_0=250$ GeV (upper) and $2E_0=500$ GeV (bottom) in continuous mode of operation (CW), see the text.
• Figure 4: Dependence of the $e^-e^-$ luminosity on the total power for $2E_0=250$ GeV (upper) and $2E_0=500$ GeV (bottom) in continuous (CW) and pulsed modes of operation, see the text.